A prior art semiconductor device including a quantum wire structure was reported in Applied Physics Letters, Vol. 55, pp.867-869 (1989). FIG. 14 is a cross-sectional view illustrating the quantum wire structure of the prior art semiconductor device. In the figure, reference numeral 100 designates a GaAs substrate having a surface in a plane that is 6.degree. off the (100) plane in the 110! ((111)A) direction. The GaAs substrate 100 has a periodic pattern of grooves 110 at the surface. The interval of the grooves 110 ranges from several microns to several tens of microns. A GaAs layer 200 is disposed on the GaAs substrate 100. The GaAs layer 200 has macroscopic steps opposite the grooves 110 of the GaAs substrate 100. A GaAs layer 300 is disposed on the GaAs layer 200 having the macroscopic steps. Portions of the GaAs layer 300 opposed to the macroscopic steps of the GaAs layer 200 are thicker than other portions. An AlGaAs layer 400 is disposed on the GaAs layer 300. A GaAs layer 500 is disposed on the AlGaAs layer 400. The AlGaAs layer 400 and the GaAs layer 500 are thicker in portions opposed to the macroscopic steps of the GaAs layer 200 than in other portions. An AlGaAs layer 600 is disposed on the GaAs layer 500.
A description is given of a method of producing the quantum wire structure.
Initially, a GaAs substrate 100 having a surface in a plane that is 6.degree. off from the (001) plane in the 110! ((111)A) direction is prepared, and a periodic pattern of grooves 110 are formed at the surface of the GaAs substrate 100. Thereafter, a GaAs layer 200 is grown on the surface of the GaAs substrate 100 by MOCVD (Metal Organic Chemical Vapor Deposition). In the crystal growth, atomic layer steps are bunched (step bunching) on the grooves 110, producing macroscopic steps at the surface of the GaAs layer 200. As a result, the GaAs layer 200 has a surface configuration in which the macroscopic steps and the (001) plane are alternatingly arranged. Thereafter, a GaAs layer 300, an AlGaAs layer 400, a GaAs layer 500, and an AlGaAs layer 600 are successively grown on the GaAs layer 200, completing a quantum wire structure as shown in FIG. 14.
A description is given of the operation of the semiconductor device having the quantum wire structure shown in FIG. 14.
Since the growth rate of GaAs is higher on the macroscopic steps than on the (001) plane, the GaAs layer 500 that is sandwiched between the AlGaAs layer 400 and the AlGaAs layer 600 has relatively thick portions 510 opposite the macroscopic steps and relatively thin portions 520 opposite the (001) plane. Therefore, the GaAs layer 500 conducts charge carriers in the relatively thick portions 510 on the macroscopic steps. As a result, in the GaAs layer 500, pseudo quantum wire structures are produced by the portions 510 extending in the direction perpendicular to the cross-section of FIG. 14.
Meanwhile, a semiconductor device including a quantum wire structure that is produced without previous patterning of a substrate is disclosed in, Japanese Journal of Applied Physics, Vol.29, pp.L483-L485 (1990) or Extended Abstracts (The 41st Spring Meeting, 1994); The Japan Society of Applied Physics and Related Societies, No. 28p-S-9. FIG. 15 is a cross-sectional view illustrating the quantum wire structure of the prior art semiconductor device. In the figure, the same reference numerals as those shown in FIG. 14 designate the same or corresponding parts. In the fabrication, initially, a GaAs substrate (not shown) having a surface in a plane that is slightly off the (001) plane in the 110! direction is prepared. When a GaAs layer 200 is grown on the surface of the substrate by MOCVD, atomic layer steps are bunched to produce a multiatomic step on the GaAs layer 200. Thereafter, An AlGaAs layer 400, a GaAs layer 500, and an AlGaAs layer (not shown) are successively grown on the GaAs layer 200 to produce an AlGaAs/GaAs/AlGaAs quantum well structure.
Also, in the quantum well structure shown in FIG. 15, since the growth rate of the GaAs layer 500 is higher on the multiatomic step than on the (001) plane, the GaAs layer 500 has a relatively thick portion 510 opposite the multiatomic step and a relatively thin portion 520 opposite the (001) plane. The relatively thick portion 510 provides a pseudo quantum wire structure for conducting charge carriers.
In the prior art semiconductor devices shown in FIGS. 14 and 15, since the quantum wire structure is produced by growing the GaAs layer 500 utilizing the dependence of the growth rate of GaAs on the surface orientation, the GaAs layer 500, that is, the relatively thin portion 520, is formed on the (001) plane of the AlGaAs layer 400. This portion 520 also provides a GaAs quantum well sandwiched by the AlGaAs layers 400 and 600. Therefore, in the quantum well structure 510, sufficient confinement of charge carriers is not realized.
Furthermore, the material of the layer 500 providing a quantum wire structure is restricted to a material having a high dependence of the growth rate on the surface orientation.
In the prior art semiconductor device shown in FIG. 14, since the substrate 100 has the periodic grooves 110 at the surface, the interval between adjacent grooves 110 is restricted by patterning of the grooves, so that the density of quantum wires within the surface of the substrate cannot be increased. In addition, the patterning of the grooves 110 complicates the fabrication process.